This invention relates to a compact keytop sheet for push-button switches suited for integration. The invention further relates to a method of manufacturing the keytop sheet.
The reduction in the size and thickness of electronic devices that has been achieved in recent years has been accompanied by a demand to reduce the size and thickness also of the keytop sheet for the push-button switches used on the control panels of these devices.
In an effort to meet this demand, a keytop sheet has been proposed in which a plurality of keytops made of synthetic resin are fixedly molded to a film sheet comprising a single film made of resin, and a frame made of synthetic resin is fixedly molded to the periphery of the film sheet so as to encircle the plurality of keytops.
A switch contact is disposed below each keytop so that if the keytop is pressed, the switch underlying it will be closed. At such time the film sheet surrounding the keytop is tensioned slightly in the downward direction.
These push-button switches using a keytop sheet of this kind include some that are used in environments in which there is the danger of humidity and moisture penetrating the interior of the switch. In such a case, there are instances in which it is preferred that the keytop sheet have a so-called waterproof structure in which humidity and moisture will not penetrate the underside of the sheet from the top side thereof. To achieve this, it is required that the film sheet be entirely devoid of holes.
When the conventional keytop sheet is such that the spacing between the keytops and the frame or the spacing between the keytops themselves is reduced for the sake of miniaturization, the following problems arise.
Specifically, FIG. 20 is a diagram showing the relationship among an angle of inclination A of a film sheet 210, L and .DELTA.L, where L represents the spacing between a keytop 220 and a frame 230 to which a film sheet 210 has been affixed, and .DELTA.L represents the amount by which the film sheet 210 must stretch when the keytop 220 is depressed by 0.3 mm.
As indicated by the graph of FIG. 20, the smaller the spacing L, the much greater the amount of stretch .DELTA.L and the angle of inclination A. In other words, the smaller the spacing L is made, the more difficult it is to depress the keytop 220. If the keytop 220 is pressed too strongly in such case, there is the danger that the film sheet 210 will be deformed.
Further, when any single keytop is pressed, a keytop alongside it is pulled slightly toward the pressed keytop. However, when the spacing between mutually adjacent keytops is made small, the amount of pull becomes much larger by reason of a principle similar to that described above in connection with FIG. 20. Consequently, when one keytop is pressed, there is the danger that the keytop alongside will be moved toward the pressed keytop to a noticeable extent.
If it is unnecessary to provide the keytop sheet itself with a waterproof function, then it will suffice to provide the portion of the film sheet surrounding each keytop with a C-shaped cut-out in order to solve the above-mentioned problem. If such an arrangement is adopted, the film sheet surrounding a keytop will not be pulled when the keytop is pressed, thereby facilitating operation of the keytop. However, when the film sheet is provided with a cut-out, the cut-out is visible from above the keytop sheet and detracts from the appearance of the device.